I don’t really follow SkyOS anymore since they gave up the idea of remaining free a few years ago. I do periodically see what Symphony and ReactOS are up to, though. And VectorLinux, DSL and Puppy Linux are worth looking into if you have some really ancient computer hardware.

I added Xubuntu and I recommend you get the text installer (its on the alternate cd) so you can install it faster. It uses the Xfce Desktop environment, which is lighter on resources than GNOME or KDE. Xfce is not ugly either, its visual style is similar to GNOME.I added Haiku.